It is commonplace in the electronic art to place alpha-numeric codes upon electronic devices for identification purposes. Historically this has been accomplished by using printing with type and ink. Means for carrying out such printing to mark devices are well-known in the art.
Recently, laser marking has been used as a replacement for printing. With laser marking, a very intense beam of light is scanned over the device surface to write out the desired markings or is projected onto the device surface through a shadow mask containing an image of the desired markings. The intense light from the laser burns, melts, ablates, or otherwise alters the surface of the device to leave a visible imprint.
Prior art laser marking techniques suffer from a number of problems and limitations, for example, visibility of the markings is often poor because of the low contrast between the irradiated areas and the non-irradiated areas. In many cases, the markings are easily damaged. For example, if marking is accomplished by burning of the surface the char or other debris formed as a result of the laser irradiation can easily rub off during subsequent handling so that the markings are lost. Further, if the laser merely produces melting or ablation, the markings can become invisible if the device is covered with a clear coating or sealer during a further stage of assembly or use, as is often the case with electronic devices.
One method which has been used to overcome these problems is to use a surface coating of a different color than the base material. The laser locally vaporizes the coating so that the darker (or lighter) base material shows through. However, this method adds additional manufacturing cost since the surface coating must be added after normal encapsulation.
Another approach has been to place a dye in the encapsulation material so that the encapsulation material itself changes color when hit by the laser. An example of this is Hysol Blue Laser Enhanced Molding Compound manufactured by the Hysol Company, Olean, N.Y. Another example is plastic molding compound containing ferric oxide. The ferric oxide containing material changes from a generally gold color to a dark brown color when irradiated by a laser.
Prior art dye loaded materials often provide less than the desired degree of color contrast after irradiation and/or suffer from color degradation when exposed to elevated temperatures for an extended period of time. Since many electronic devices must withstand elevated temperatures for hundreds to thousands of hours during testing and use it is highly desirable to have materials which not only provide good marking visibility, but are also stable. A further consideration which is particularly important for electronic devices is that the coloring agents or dyes must not add alpha particle sources or mobile ion sources to the encapsulation material since these are known to cause severe degradation of electronic devices, particularly semiconductor devices.
Accordingly, it is an object of the present invention to provide an improved material adapted for laser marking of electronic devices.
It is a further objective to provide an improved material for laser marking of electronic devices which gives a high degree of color contrast when laser irradiated, as compared to the non-irradiated portion of the material.
It is an additional objective of the present invention to provide an improved material adapted for laser marking of electronic devices which shows less color change or contrast degradation with time and temperature as compared to prior art materials.
It is a further object of the present invention to provide this improved laser marking material without introducing mobile ions and/or alpha particle sources.
It is a still further object of the present invention to provide this improved laser marking material using raw materials which are inexpensively and readily obtained free of mobile ion and alpha particle contamination.
It is an additional object of the present invention to provide an improved method for forming electronic devices adapted for laser marking using the above-mentioned material.
It is a further object of the present invention to provide an improved method for laser marking electronic devices using the above-mentioned material.
It is an additional object of the present invention to provide improved electronic devices utilizing the above-mentioned laser markable material and method.